1. Field of the Invention
This invention relates to digital sound systems for motion picture films.
2. Description of the Prior Art
Current motion picture films employ analog sound recording techniques. Most motion pictures use an analog optical sound track that is printed onto the film along with the picture, and is optically scanned to reproduce the sound. Another technique is to record the sound onto magnetic strips that are coated along the edges of the film; this process is used for example in producing "Dolby.RTM. 70 mm Sound".
Optical sound tracks can become dirty and produce a noisy output. Magnetically recorded sound tracks are subject to noise problems similar to those encountered with other audio tapes. It would be highly desirable to upgrade the quality of motion picture sound to a level comparable to that achieved with a compact disc.
The majority of multitrack motion picture theaters are designed to exhibit Dolby stereo-optical films. This is a two track optical sound format that provides left and right analog sound tracks with derived center and surround tracks. The sound tracks for this format are recorded as two "dual bilateral" variable area tracks in the conventional optical track area of the motion picture release print. During the motion picture post production sound mixing process the sound tracks are mixed through a two-track matrixing device that allows the sound mixers to hear exactly how the left, right, and derived center and surround tracks will sound in a stereo-optical theater; this is the current standard method of mixing motion picture sound tracks.
Although the dual tracks are recorded using noise reduction techniques such as Dolby type A, they have a dynamic range that is significantly less than that of magnetic or digital tracks. The dual tracks are also susceptible to all of the well known failings of optical sound tracks, including a limited frequency response and a susceptibility to noise caused by dust on the film. In addition, the film exposing and developing steps used to produce the optical sound tracks are quite variable, resulting in an optical sound track quality that can vary dramatically. However, the majority of all current motion pictures are released with this standard type of sound track.
It has previously been proposed to replace the analog optical sound track with a digital sound track that digitally encodes the sound. Although theoretically this could be used to give a higher quality sound reproduction, it would be quite expensive to both record the numerous small digital data bits and to provide equipment for reading them back, and the tightly compacted data could give rise to its own noise problems. In addition, it would be difficult to reliably print such digital tracks using conventional film laboratory methods. There would also be a limit to the number of sound tracks that could be encoded onto the film. Since the digital track preempts the normal analog optical sound track two kinds of prints would have to be made and distributed, one with a digital sound track and the other with an analog sound track; if a digital print were sent to a theater without digital play capability, it could not be shown. Furthermore, a theater playing a digital print would have no backup in case of a failure of the digital reader.